Silver halide is an inorganic photochromic material known from old times as a material causing the so-called photochromic phenomenon according to which a color reversibly changes depending on the presence or the absence of light. Although capable of exhibiting a high sensitivity to coloring and to decolorizing and a high resistance to light, silver halide induces only a narrow range of color change in respect of limited kinds of hue and is difficult to use in combination with other materials than inorganic glass. With these drawbacks, silver halide has been used for extremely limited applications, for example, as lenses for spectacles, automotive sunroof materials, etc.
In recent years, there have been developed numerous photochromic materials comprising organic photochromic compounds which induce a broad range of color change in respect of extended kinds of hue and which are highly compatible with synthetic resins and with various organic compounds. These organic photochromic materials are expected to find a wide range of applications. Known as such organic photochromic compounds are azobenzene-type compounds, thioindigo-type compounds, dithizone metal complexes, spiropyran-type compounds, spirooxazine-type compounds, fulgide-type compounds, dihydropyrene compounds, spirothiopyran-type compounds, 1,4-2H-oxazine, triphenylmethane-type compounds, viologen-type compounds, pyran-type compounds, etc. Of these compounds, spiropyran-type compounds, spirooxazine-type compounds and pyran-type compounds are excellent in the sensitivity to coloring and in the color density. Strenuous efforts have been made to alleviate the conventional problems that these compounds involve a low rate of color reversion and reduce the color density or decolorize at a temperature higher than room temperature. As a result, the compounds substantially free of these shortcomings have been prepared. However, these organic photochromic compounds are so low in the stability to light that the compounds may decompose in a few days and become non-responsive to light, when coloring or decolorizing repeatedly occurs on intermittent irradiation or are subjected to continuous irradiation in air.
To obviate this serious drawback, various attempts have been made by incorporating additives into the organic photochromic compound with the effects already disclosed. Heretofore disclosed as such additives are hindered phenol-type, phosphite-type and thioether-type antioxidants, ultraviolet absorbers, singlet oxygen quenchers, nickel-type metal complexes and hindered amine-type compounds. Yet the use of additives other than hindered amine-type compounds produce substantially no effect. When used to enhance the light resistance of organic photochromic material, a hindered amine-type compound has been added to a high-molecular-weight synthetic resin matrix along with the organic photochromic material to provide a resin matrix. In this case, the hindered amine-type compound is used in the form as incorporated in the resin matrix or in a coating composition or a printing ink containing the resin matrix dissolved or dispersed in a suitable organic solvent. The organic photochromic material containing the hindered amine-type compound in this form remains insufficient in the resistance to light although higher in this property than when free of a hindered amine-type compound, and is low in the sensitivity to coloring and in the rate of color reversion and poor in the color density because the organic photochromic compound is present as dispersed in the resin. While an attempt was made to lessen the drawbacks by use of a plasticizer, the use of plasticizer reduces the resistance to light, resulting in failure to achieve the contemplated object. Since the hindered amine-type compound used in the above manner exists in the mixture containing the resin, and is less compatible with the resin, the excess compound would be separated out if an increased amount thereof is used. For this reason, the hindered amine-type compound has been used in an amount equal to or less than that of the organic photochromic material.